Half of mean median life expectancy which is something like 40 years. Same for everyone. Makes no sense that one can write a book and then live off it forever. That does not encourage more books. Good gig though.
Edit: Ideally something better would be good but we only know copyright.
Someone losing copyright of their book doesn't necessarily mean they're going to write more. A vast majority of published writers cannot live off of a single book (a lot can't from multiples). It's actually incredibly hard to make a living off writing.